1. Field of the Invention
The present invention relates to a satellite broadcast receiving converter having a waveguide which is loaded on an outdoor antenna apparatus for receiving two kinds of linearly polarized wave signals.
2. Description of the Related Art
A conventional satellite broadcast receiving converter is described with reference to FIGS. 6 through 9. Here, FIG. 6 is a lateral view in cross section of the conventional satellite broadcast receiving converter, FIG. 7 a frontal view of the same, FIG. 8 a rear view illustrating the internal construction of the same, and FIG. 9 an external view of the same.
In these figures, a waveguide 30 is formed into a cylindrical form both ends of which are open. A circuit board 31 formed with a microstrip line is provided at the rear end of the opening 30a for extension while a metallic bottomed case 32 having a jaw portion 32a is disposed at a position, where the end of the opening 30a is closed with a lid, by way of the circuit board 31. Further, within the waveguide 30, disposed approximately 1/4 wavelength of the received electric wave (the frequency bandwidth ranges approximately from 10.7 GHz to 12.75 GHZ) ahead of the rear circuit board 31 is a first probe 33 for detecting a first linearly polarized wave (for example, horizontally polarized wave). This first probe 33 is of substantially L-shape, and its proximal end portion is connected to the circuit board 31 while its portion extending linearly from the proximal end portion is covered with an insulative member 34 made of, for example, Teflon to incorporate into a recessed groove 30b of the waveguide 30 in such a way that its tip end portion may protrude into the waveguide 30 by a predetermined size.
Of both surfaces (front and rear) intersecting at a right angle with the axial line of the waveguide 30, on the surface at the side of the first probe 33, a short-circuit pattern 35 is provided to make the first probe 33 detect the reflected first linearly polarized wave while, on the other surface, a second probe 36 is patterned to detect a second linearly polarized wave (for example, perpendicularly polarized wave) intersecting at a right angle with the first linearly polarized wave. Here, since the circuit board 31 is negligibly thin as compared with the wavelength of the received electric wave, after all, any of the short-circuit pattern 35 and the second probe 36 is positioned approximately 1/4 wavelength separate from the first probe 33 in the direction in which the electric wave travels (in the direction of arrow VII). Further, in this embodiment, the internal bottom surface of the metallic case 32 is formed with a short-circuit surface 32b to detect the reflecting second linearly polarized wave by the second probe 36.
Incidentally, within the circuit board 31, a processing circuit is provided in which the signal detected by the first probe 33 and the second probe 36 is appropriately processed (amplified or converted in frequency), and the first probe 33 and the second probe 36 are each connected to first stage amplifying transistors 41, 42 by way of withdrawing patterns 39, 40 on the circuit board 31, as shown in Fig, 8. Further, provided on the metallic case 32 are escape recesses 32c, 32d to previously avoid contact with these withdrawing patterns 39, 40.
Further, the first stage amplifying transistor 41 is connected to a second stage amplifying transistor 45 by way of the withdrawing pattern 43 while, likewise, the first stage amplifying transistor 42 is connected to the second stage amplifying transistor 45 by way of the withdrawing pattern 44. Either one of the first stage transistors 41, 42 operates depending on which one of the two linearly polarized waves is received. That is, when the first linearly polarized wave is received, the first stage amplifying transistor 41 operates, and when the second linearly polarized wave is received, the first stage amplifying transistor 42 operates. Thus, either of the linearly polarized waves is transfered to the second stage amplifying transistor 45.
The portion of the circuit board 31 which is located within the waveguide 30 is formed into a substantially T-shaped form by providing a notch 31b, where the short-circuit pattern 35 and the second probe 36 are formed. That is, provision of the notch 31b is allowed for so that the electric wave (the second linearly polarized wave) detected by the second probe 36 is not attenuated.
On the other hand, at the portions of both front and rear surfaces of the circuit board 31 which are opposed to the periphery of the end 30a of the rear opening of the waveguide 30, a ground electrode 37 comprising a soldered layer is provided. These ground electrodes 37, 37 are each connected to each other by a plurality of through holes 31a for electrical conduction of both front and rear surfaces which are provided through the circuit board 31 while the short-circuit pattern 35 is connected to the ground electrode 37. Further, since the jaw portion 32a of the metallic case 32 is fixed to the periphery of the opening end 30a of the waveguide 30 by way of the circuit board 31 by means of a vis 38, the waveguide 30 and the metallic case are each press-fitted to the ground electrode 37 on both surfaces of the circuit board 31. Incidentally, the circuit board 31 and the metallic case 32 which are attached to the rear portion of the waveguide 30 are located within a casing 46 which houses the circuit to cover by means of a cover 47. As shown in FIG. 9, an output connector 48 is provided to protrude from this casing 46 outwardly to emit the received signal.
Incidentally, since the waveguide 30 is formed into a cylindrical form, the distribution of the electromagnetic field of the electric wave which propagates therein takes mainly the TE11 mode. However, in reality, due to the presence of the discontinuous points caused by physical size variation of the waveguide or of the circuit board 2, the TM01 mode also occurs, which allows only about 25 dB isolation between the first and second linearly polarized waves to be inadequately obtained. That is, at the first probe 33 for detecting the first linearly polarized wave, a second linearly polarized wave is detected and, at the second probe 36 for detecting the second linearly polarized wave, the first linearly polarized wave is detected.
In addition, since the transmission loss of the received electric wave which propagates through the waveguide 30 increases at the frequency (for example, 9 GHz) lower than the frequency bandwidth (10.7 GHz-12.75 GHz) of the electric wave which is entered to the waveguide 30 (the waveguide exhibits the performance of a bypass filter), the isolation is further decreased, and if the frequency becomes lower, then the amplification of the first stage amplifying transistors 41, 42 becomes higher, which causes the first probe 33, withdrawing pattern 39, first stage amplifying transistor 41, withdrawing patterns 43, 44, first stage amplifying transistor 42, withdrawing pattern 40, and the second probe 36 to form a closed loop to result in a large oscillation.
Accordingly, a satellite broadcast receiving converter according to the present invention may eliminate the unnecessary TM01 mode electromagnetic field to make the isolation between the first and second linearly polarized waves greater to thereby prevent occurrence of the oscillation.